Serotonin-binding proteins in the bovine cerebral cortex: interaction with serotonin and catecholamines

The soluble serotonin-binding proteins (SBP) present in bovine frontal cortex are very similar to those reported in rat brain. Binding of [³H]serotonin to SBP, present in ammonium sulphate-precipitated proteins from bovine cortex, requires Fe²⁺ but not Fe³⁺. In the presence of an optimal concentration of Fe²⁺ (0.1 mM), bovine SBP behave as a single class of non-cooperative sites for [³H]serotonin binding (B_max = 120 ± 12 pmol/mg protein, K_D = 0.12 ± 0.04 μM, n = 3). Binding of [³H]serotonin is decreased by nucleotides and by reagents which modify sulfhydryl groups and reduce disulfide bonds and by metal ion chelators. Serotonin analogs possessing an hydroxyl group on the indole ring and catecholamine analogs possessing an intact catechol moiety are effective competitors (K_i from 0.1 to 0.3 μM). In both cases, the aliphatic amino group does not contribute to the binding, but the affinity is strongly decreased if aromatic hydroxyl groups are methoxylated. Catecholamine-SBP interactions can also be demonstrated directly by binding experiments. Binding of [³H]dopamine is greatly enhanced by Fe²⁺, Cu²⁺ and Mn²⁺, but not by Fe³⁺. The Fe²⁺-dependent binding component of [³H]dopamine is saturable (B_max = 279± 64 pmol/mg protein, K_D = 0.19 ± 0.02 μM, n = 3), and possesses the same physicochemical properties as SBP: it elutes immediately after the void volume on a Sephacryl S100 HR (1.6 × 140 cm) gel filtration column (reflecting aggregation) and it migrates with an apparent molecular weight of 57–58 kDa on native polyacrylamide gel electrophroresis. Whereas the serotonin-storing role of SBP in serotonergic neurons has already been well documented, the present data advocate that these proteins may also possess catecholamine-storing properties.     

Identification of non-serotonergic [3H]ketanserin binding sites on human platelets and their role in serotonin release

[³H]Ketanserin was found to label (besides a low amount of 5-HT₂ receptors) non-serptonergic binding sites on human platelet membranes. The latter binding was detected in the presence of excess of the 5-HT₂ antagonist BW501, and was potently inhibited by tetrabenazine. [³H]Ketanserin revealed a K_D value = 19 ± 4 nM and a B_max = 425 ± 82 fmol/10⁹ platelets for these binding sites. [³H]Ketanserin binding in the presence of BW501 was inhibited by the tetrabenazine derivative RO-4-1284 (IC₅₀ = 1.4 nM), tetrabenazine (IC₅₀ = 8.6 nM) and the ketanserin derivatives R 71 278 (IC₅₀ = 6.3 nM). R 47 288 (IC₅₀ = 17 nM) and R 71 428 (IC₅₀ = 100 nM). Ketanserin revealed an IC₅₀ = 32 nM. The drugs were found to trigger the release of ³H from [³H]5-HT-loaded human platelets in superfusion experiments in vitro. The amount of ³H released by the drugs correlated with their binding affinities for the non-serotonergic sites. The non-serotonergic [³H]ketanserin binding sites on human platelets and their possible role in triggering monoamine release corresponded to the properties of non-serotonergic ketanserin binding sites previously characterized in rat striatum. The possible role of the action of ketanserin on the non-serotonergic sites in the reported partial reduction by ketanserin of the monoamine content in cardiovascular tissues is discussed.     

Decrease in [3H]-serotonin binding in rat brain produced by the repeated administration of either monoamine oxidase inhibitors or centrally acting serotonin agonists

The effect of repeated administration of monoamine oxidase inhibitors or serotonin agonists on [³H]-serotonin binding and serotonin concentrations in rat cerebral cortex was examined. Five days of clorgyline administration, which alone caused a significant reduction in [³H]-serotonin binding, was unable to do this in animals pretreated with drugs that deplete brain serotonin. Pretreatment of rats with alpha-methyl-paratyrosine did not block the clorgyline-induced reduction in [³H]-serotonin binding. Repeated treatment of rats with nialamide also lowered [³H]-serotonin binding and the return of binding sites to control values was correlated temporally with the return of the serotonin concentration to control values. Repeated administration to rats of the serotonin agonists, quipazine or TFMPP, produced a decrease in [³H]-serotonin binding. Both the clorgyline and serotonin agonist-induced reduction in [³H]-serotonin binding were due to a significant reduction in the maximum specific binding capacity with no change in the apparent binding constant. The data presented here are consistent with the hypothesis that MAO inhibitors reduce [³H]-serotonin binding indirectly, by increasing over time the exposure of serotonin receptors to the indolealkylamine.     

Binding of 2′,5′-dideoxyadenosine to brain membranes: Comparison to P-site inhibition of adenylate cyclase

Membranes from rat cerebral cortex and striatum contain a relatively large number of high-affinity binding sites for [³H]2′,5′-didepxyadenosine, [³H]adenine arabinoside, and [³H]adenosine. The binding of [³H]2′,5′-dideoxyadenosine and [³H]adenine arabinoside was virtually unaffected by relatively specific agonists and antagonists for adenosine receptors, such as 2-chloroadenosine, N⁶-phenylisopropyladenosine or theophylline. Binding of [³H]adenosine was partially blocked by such receptor ligands. The specific binding of all three ligands was antagonized by a variety of adenosine analogs which inhibit adenylate cyclase by interaction with the so-called P-site associated with this enzyme. However, potencies of adenosine analogs as P-site inhibitors of adenylate cyclase and as antagonists of binding do not correlate well. 5′-Methylthioadenosine had high potency and efficacy versus binding of [³H]2′,5′-dideoxyadenosine but had virtually no effect on activity of adenylate cyclase. 2-Fluoroadenosine was less potent than adenosine as an antagonist of specific binding of [³H]2′,5′-dideoxyadenosine, which 2-fluoroderivatives of adenosine, adenine arabinoside and adenine xylofuranoside were more potent than the parent compounds as P-site inhibitors. The significance of the binding sites for [³H]2′,5′-dideoxyadenosine remains unclear, but their presence complicates the use of [³H]adenosine and certain analogs as ligands for adenosine membrane sites associated with adenylate cyclase.     

Further characterization of [3H]ifenprodil binding in rat brain

The present study was undertaken to characterize [³H]ifenprodil binding in rat brain. [³H]ifenprodil showed saturable, high-affinity binding at 4°C. Specific binding, defined with 10 μM ifenprodil as a competitor, was inhibited biphasically by the s receptor ligands, GBR 12909, 1,3-di-o-tolylguanidine (DTG), and (+)-3-(3-hydroxyphenyl)-N-propylpiperidine ((+)-3-PPP). At 4°C, 3 μM GBR 12909, which inhibited about 50% of specific binding of [³H]ifenprodil, was used to mask σ receptors. Under these conditions, specific binding of [³H]ifenprodil was inhibited potently by ifenprodil, SL 82.0715, poly(l-arginine), poly(l-lysine), neomycin, ruthenium red, spermine, arcaine and spermidine. In the presence of 3 μM GBR 12909, Zn²⁺ and Mg²⁺ partially inhibited specific binding of [³H]ifenprodil at 4°C. In contrast, in the absence of GBR 12909, at 37°C specific binding of [³H]ifenprodil was partially inhibited by Zn²⁺, but not by Mg²⁺. The anatomical distribution of [³H]ifenprodil binding at 4°C (GBR 12909 included) in rat brain closely paralleled that of [³H]MK-801 (dizocilpine) binding (r = 0.971, P < 0.005). Without GBR 12909, specific [³H]ifenprodil binding at 37°C was inhibited potently by σ ligands. In the presence of 3 μM GBR 12909, [³H]ifenprodil binding at 4°C was highest in synaptosomal and myelin fractions; however, without GBR 12909, [³H]ifenprodil binding at 37°C was highest in microsomal and myelin fractions, consistent with the subcellular distribution of σ receptors. The results suggest that, in the presence of 3 μM GBR 12909, at 4°C, [³H]ifenprodil binds to sites that are sensitive to polyamines and related compounds; and that without GBR 12909, at 37°C, [³H]ifenprodil interacts with σ receptors in rat brain.     

Novel serotonin receptors in Fasciola: Characterization by studies on adenylate cyclase activation and [3H]LSD binding

Serotonin (5-HT) receptors coupled to adenylate cyclase [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1] in the liver fluke Fasciola hepatica have been characterized by adenylate cyclase activation studies and by direct binding studies using $\text{[}{}^3\text{H}\text{]-d-}\text{lysergic acid}$ diethylamide ([³H]LSD) as a radioligand. Inhibition of 5-HT stimulation of adenylate cyclase by a series of 5-HT antagonists revealed a potency order of LSD = 2-bromo-LSD > methiothepin > metergoline = cyproheptadine > methysergide > spiroperidol. [³H]LSD binding to a cell-free fluke particle preparation was rapid, stereospecific, and proportional to protein concentration. Scatchard analysis indicated multiple binding sites which, when resolved into two components, gave for the high affinity site an apparent dissociation constant of 25 nM and a receptor concentration of 160 fmoles/mg protein. The ability of a series of compounds to compete for [³H]LSD binding sites correlated closely with their ability to inhibit 5-HT stimulation of adenylate cyclase. [³H]LSD binding sites were most concentrated in the anterior region of the fluke which was consistent with the higher levels of 5-HT activated adenylate cyclase found in this region. GTP and 5′-guanylyl imidophosphate, a poorly hydrolyzable GTP analog, decreased the affinity of the agonist 5-HT for the binding sites but had little effect on the affinity of the antagonist 2-bromo-LSD. Calcium at concentrations above 300 μM significantly reduced both [³H]LSD binding and 5-HT activation of adenylate cyclase. The results indicate that [³H]LSD can be used to label the 5-HT receptors coupled to adenylate cyclase activity. The pharmacological specificity and other characteristics of the fluke receptors appear to differ from the properties of reported mammalian 5-HT receptors. As a result, serotonin receptors in the flukes represent sites that may be amenable to selective manipulation by new chemotherapeutic agents useful in the treatment of these parasite infections.     

The effect of psychoactive drugs on beta-adrenergic receptor binding sites in rat brain

The effect of a variety of psychotropic drugs, given to rats repeatedly over 16 days, on the number of beta-adrenergic receptor binding sites in cerebral cortex was measured. Labelled dihydroalprenolol, [³H]-DHA, was used to measure beta-adrenergic binding sites. Both tricyclic antidepressants (amitriptyline, chlorimipramine, desmethylimipramine and nortriptyline) and monoamine oxidase inhibitors (nialamide and tranylcypromine) lowered [³H]-DHA binding significantly. Repeated treatment of rats with the antidepressant. iprindole. produced the same effect as did treatment with bupropion. However, the experimental antidepressant mianserin did not reduce [³H]-DHA binding nor did 11 other psychoactive compounds including chlorpromazine, diazepam, l-DOPA, tripelennamine and cocaine. The reduction in [³H]-DHA binding produced by nialamide or iprindole treatment was due to a reduction in the maximum number of beta-adrenergic receptor binding sites. Drug treatment-induced lowering of [³H]-DHA binding sites in cerebral cortex appears to have utility as a pre-clinical test for antidepressant drugs.     

Polyamines allosterically modulate [3H]nitrendipine binding to the voltage-sensitive calcium channel in rat brain

The effects of polyamines on radioligand binding to the slow voltage-dependent Ca²⁺ channel were studied using membranes from the rat cerebral cortex. [³H]Diltiazem binding was inhibited by arcaine (IC₅₀ = 55 μM) and, in decreasing order of potency, by agmatine, spermidine, spermine and putrescine. Under control conditions, only spermidine and spermine allosterically inhibited [³H]nitrendipine binding while arcaine, agmatine and putrescine were inactive. Nevertheless, putrescine antagonized the effect of spermine as well as the allosteric effects of diltiazem and verapamil on the binding of [³H]nitrendipine, in a manner analogous to that shown previously for Ca²⁺. Thus, polyamines may function as endogenous modulators of the voltage-dependent Ca²⁺ channel.     

Guanine nucleotide and cation regulation of radioligand binding to R i adenosine receptors of rat fat cells

Summary The modulation of radioligand binding at R _i adenosine receptors of rat fat cells by guanine nucleotides and cations was investigated. Guanine nucleotides (in the order of potency: GTP=GDP>Gpp(NH)p>5-GMP) decreased the binding of the R _i receptor agonist (–)N⁶-phenylisopropyl[³H]adenosine ([³H]PIA), but did not affect binding of the antagonist 1,3-diethyl-8[³H]phenylxanthine ([³H]DPX). Saturation of [³H]PIA binding revealed that GTP (100 mol/l) converts the high affinity form of the R _i receptor into a low affinity form. This effect was confirmed in kinetic experiments. GTP decreased the potency of agonists in competing for [³H]DPX binding, as shown by a 50-fold shift of the K _i-value for (–)PIA, whereas antagonist-induced inhibition of binding remained unchanged. The divalent cations Mg²⁺ and Ca²⁺ produced a slight increase in [³H]PIA binding but did not affect [³H]DPX binding. Mn²⁺ markedly decreased both agonist and antagonist binding at R _i adenosine receptors. Divalent cations reversed the guanine nucleotide-induced decrease of affinity of the R _i receptor. Na⁺ did not significantly affect agonist or antagonist binding but abolished the stimulatory effect of Mg²⁺ on agonist binding in the presence of GTP. Our data indicate that guanine nucleotides convert the R _i adenosine receptor of rat fat cells from a high to a low agonist affinity state and that the modulation of radioligand binding by mono-and divalent cations differs from that of R _i receptors of other tissues.     

Properties of the binding sites of [3H]9-methyl-7-bromoeudistomin D in bovine aortic smooth muscle microsomes

Abstract— [³H]9-Methyl-7-bromoeudistomin D ([³H]MBED), a powerful caffeine-like Ca²⁺ releaser, binds to the caffeine binding site of terminal cisternae of skeletal muscle sarcoplasmic reticulum and activates Ca²⁺-induced Ca²⁺ release. Properties of the binding site of [³H]MBED were investigated in aortic smooth muscle. The specific activity was higher in microsomes than in other fractions. [³H]MBED binding sites in smooth muscle microsomes were of a single class with a high affinity (K_D 50 Nm ), comparable with that in skeletal muscle sarcoplasmic reticulum. Caffeine competitively inhibited [³H]MBED binding, indicating MBED shares the same binding site with caffeine. Solubilization and fractionation of the microsomes gave two fractions of [³H]MBED binding activities. These results suggest that, in smooth muscle, there are multiple binding sites of [³H]MBED and caffeine, which might correspond to different pharmacological actions of caffeine on smooth muscle. Therefore, [³H]MBED, which binds to the different binding sites of caffeine, is useful as a probe for investigation of the actions of caffeine at the molecular level.     

Cocaine binding sites in fetal rat brain: implications for prenatal cocaine action

Binding of [³H]cocaine to membrane preparations from whole fetal rat brain was studied. High-affinity binding (10 nM cocaine) was detected as early as gestational day (GD) 15 and steadily increased across subsequent development. Saturation studies comparing [³H]cocaine binding at GD20 and adulthood yielded similar K_D values, and LIGAND analyses favored a two-site model if an extended range of [³H]cocaine concentrations was used. Various monoamine uptake inhibitors displaced labeled cocaine with potencies consistent with the idea that [³H]cocaine labels the dopamine (DA), serotonin (5-HT), and possibly also the norepinephrine (NE) transporters in whole fetal brain preparations. Synaptosomal DA uptake was well developed by GD20, as was the potency of cocaine to inhibit such uptake. The results indicate that functional, monoamine transporter related cocaine binding sites are present in the fetal rat brain. Such sites are likely to play an important role in mediating the direct interactions of prenatally-administered cocaine with developing monoaminergic systems in both animals and humans.Some of these data were published in preliminary form in: Meyer JS (1992) Prenatal neurochemistry of cocaine. Ann NY Acad Sci 654:487–488     

Treatment with trazodone plus phenoxybenzamine accelerates development of decreased type 2 serotonin binding in rat cortex

Combined administration of phenoxybenzamine with the antidepressant trazodone for four days produced a decrease in rat cerebral cortical type 2 serotonin binding as measured with [³H]spiperone. Trazodone alone had no effect on type 2 serotonin binding or β-adrenergic binding as measured with [³H]dihydrolprenolol, during this same time period.     

The binding of [3H] pargyline to rat liver mitochondrial monoamine oxidase

The synthesis and purification of tritium labelled N-desmethylpargyline and pargyline are described. The suitability of these irreversible suicide inhibitors of monoamine oxidase (MAO) as ligands in binding studies to rat liver mitochondrial MAO has been evaluated. [³ H] Pargyline was found to be more satisfactory than its N-desmethyl analogue because of its greater potency and lower proportion of non-specific binding. The binding of pargyline reached saturation when about 31 pmol mg protein^?1 was bound. It was not possible to explain the time course of the binding by either simple first or second-order kinetics. [³ H]-Pargyline is a potentially valuable ligand for the estimation of the concentration of MAO active centres without the need to remove the enzyme from the mitochondrial membrane.     

Platelet serotonin and [3H]paroxetine binding correlate with recurrence of suicidal behavior

To distinguish state-from trait-dependent associations between serotonergic function and suicidal behavior, platelet serotonergic measures were repeatedly measured, during a 1-year follow-up, in 106 patients who had recently attempted suicide for at least a second time. A major DSM-III-R axis I diagnosis or use of antidepressants were reasons for exclusion. A higher affinity constant (K_D) of platelet [³H]paroxetine binding was related to a higher risk of short-term recurrence of a suicide attempt, suggesting a state relationship. Higher levels of platelet serotonin at baseline were a significant predictor of a recurrent suicide attempt within the year of follow-up, suggesting a trait relationship. These associations held equally within the subgroup of 73 patients with a borderline personality disorder. Neither the maximum number of binding sites (B_max) of [³H]paroxetine nor platelet monoamine oxidase activity correlated with suicidality. The observed association between indicators of platelet serotonin uptake and suicidal behavior suggests a state-and trait-dependency between suicidality and central serotonergic dysfunction. Received:3 December 1996 / Final version: 6 February 1997     

Neuroleptic drug interactions with norepinephrine alpha receptor binding sites in rat brain.

Alpha adrenergic receptor sites in mammalian brain tissue can be labeled by the binding of [³H]WB-4101 (2-([2′,6′-dimethoxy] phenoxyethylamino) methyl benzodioxan), a potent α-adrenergic antagonist. Numerous neuroleptic drugs of phenothiazine, butyrophenone and thioxanthene classes are potent in competing for [³H]WB-4101 binding, with affinities resembling those of classic α-antagonists such as phentolamine and phenoxybenzamine. The potencies of neuroleptics in competing for WB-4101 binding sites correlate closely with their potencies in antagonizing norepinephrine and epinephrine induced lethality in rats, confirming that affinity for WB-4101 binding sites predicts α-receptor antagonism in vivo. The relative affinities of neuroleptics for WB-4101 binding sites and for dopamine receptors as labeled by [³H]haloperidol provides an index of the relative propensities of these drugs for eliciting autonomic side effects such as orthostatic hypotension and sedation.     

Unusual binding of [3H] MDL 72222 in guinea pig hippocampus

[³H] MDL 72222 labeled a non-homogeneous population of sites in guinea pig hippocampal membranes (K_d1 = 1 nM; K_d2 = 60 nM). The binding was not sodium dependent. Competition studies with a variety of characterizing agents showed displacement of [³H] MDL 72222 binding by 5-HT uptake inhibitors. [³H] MDL 72222 binding was not effectively displaced by established 5-HT₃ antagonists. MDL 72222, fluoxetine, fluvoxamine and citalopram competitively inhibited the uptake of [³H] 5-HT into guinea pig hippocampal synaptosomes with K_i values of 1.97, 0.02, 0.023, 0.049 μM, respectively. The results demonstrate that [³H] MDL 72222 labels a non-homogeneous population of sites in guinea pig brain, as well as inhibiting 5-HT uptake into synaptosomal preparations.     

Enhancement of benzodiazepine binding by progabide (SL 76002) and SL 75102

The new GABA derivatives, progabide (SL 76002) and SL 75102 (the corresponding carboxylic acid), enhanced [³H]diazepam binding to rat cortical membranes. SL 75102 was as effective as GABA, and both were less active than muscimol. Similar to other GABA agonists, progabide and SL 75102 enhanced binding in vitro by an increase in affinity. The elevation of [³H]diazepam binding in the presence of SL 75102 was antagonized by bicuculline. In ex vivo experiments, brain membrane preparations of mice pretreated with progabide were found to bind a greater amount of [³H]diazepam. This increased binding was characterized by an elevation in B_max, while K_D was unaffected. [³H]Flunitrazepam binding to brain in vivo was also enhanced in mice pretreated with progabide. Other GABA agonists (muscimol, THIP, isoguvacine) and GABAergic agents (sodium valproate) elicited an increase in [³H]flunitrazepam binding in vivo. Binding of intravenous [³H]flunitrazepam in vivo may be useful for discovering enhancers of benzodiazepine binding.     

Short-term lithium administration to healthy volunteers produces long-lasting pronounced changes in platelet serotonin uptake but not imipramine binding

Platelet [³H]-5HT uptake, [³H]-imipramine binding and endogenous 5HT levels were measured in healthy volunteers during short-term (20 days) administration of lithium, and following its withdrawal. The V _max of [³H]-5HT uptake was significantly decreased during lithium treatment. Following lithium withdrawal, platelet [³H]-5HT uptake (V _max) remained decreased and was followed by a pronounced rebound effect in some of the subjects for up to 3 months. The affinity constant (K _m) of [³H-5HT uptake was not modified. Binding of tritiated imipramine during the same period and platelet 5HT levels measured till 14 days after withdrawal was not affected by lithium treatment. As lithium is devoid of in vitro effects on both 5HT uptake and imipramine binding, it is concluded that the effects of lithium on the 5HT transporter do not reflect a direct effect on the transporter complex. Our results indicate that lithium-induced changes at the level of 5HT uptake in platelets are not correlated with concomitant variations in platelet 5HT content and can be dissociated from modifications at the level of imipramine binding sites within the macromolecular complex of the 5HT transporter. Moreover, platelet 5HT uptake is apparently modulated by lithium, with a similar pattern in healthy volunteers and in manic-depressive patients.     

Characteristics of [3H]fentanyl binding to the opiate receptor

The present study characterises the binding of the highly lipophilic opiate agonist [³H]fentanyl to homogenates of the rat central nervous system. At 25°C, association of [³H]fentanyl with its binding site was rapid ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 2.5}\text{min}$). Dissociation from the binding site was biphasic ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{'}\text{s}\text{= 4.0}\text{and}\text{100}\text{min}$) suggesting the existence of high and low affinity binding sites. Scatchard plots of saturation isotherms were curvilinear, confirming the presence of high (K_D = 0.46 nM) and low K_D = 4.26 nM) affinity binding sites. Increasing temperature and the concentration of sodium ion decreased the [³H]fentanyl binding. Opiate agonists, antagonists and mixed agonist-antagonists were all potent (IC₅₀'s < 20 nM) in displacing [³H]fentanyl and displacement by levorphanol and dextrorphan indicated that [³H]fentanyl binding was stereospecific. The μ and δ selective peptides, morphiceptin and [D-Ala²,D-Leu⁵]enkephalin, had IC₅₀ values of 87 and 9.2 nM respectively. The regional distribution of [³H]fentanyl binding was in the rank order striatum ? midbrain > hypothalamus > cortex > hippocampus > brainstem > spinal cord > cerebellum. Comparison of [³H]fentanyl, [³H]naloxone and [³H-d-Ala², d-Leu⁵]enkephalin binding in the hypothalamus-thalamus (μ-enriched) compared with the frontal cortex-striatum (δ-enriched) indicated that the pattern of [³H]fentanyl labelling was similar to that obtained with [³H]naloxone, but differed from that obtained with [³H-d-Ala²,d-Leu⁵]enkephalin. These characteristics suggest that [³H]fentanyl binds to the μ-opiate receptor. These findings are discussed in relation to the high lipid solubility of fentanyl as compared with morphine.     

Apomorphine‐Induced Aggressive Behaviour in para‐Chlorophenylalanine‐Treated Male Rats: Implications to Brain [3H]Ketanserin Binding and Monoamine Content

Abstract: The aim of this study was to investigate the role of serotoninergic neurotransmission and the effect of acute para‐chlorophenylalanine (350 mg/kg, intraperitoneally) treatment on apomorphine‐induced aggressive behaviour in adult male Wistar rats. In addition, [³H]ketanserin binding and monoamine content were studied. Repeated administration of apomorphine (1.0 mg/kg, subcutaneously, once daily for two weeks) gradually induced aggressive behaviour. Acute p‐chlorophenylalanine treatment down‐regulated the [³H]ketanserin binding and reduced over 90 per cent the content of serotonin and 5‐hydroxyindolacetic acid. In a half of the p‐chlorophenylalanine‐treated animals, the aggressive behaviour was suppressed, while there was no difference in [³H]ketanserin binding or monoamine content between the p‐chlorophenylalanine treated aggressive and non‐aggressive animals. In conclusion, the acute p‐chlorophenylalanine treatment attenuates the aggressiveness only in half of the animals, while the latter phenomenon is independent on the CNS monoamine content or [³H]ketanserin binding.